This quest is not available in game.
Use the Brazier of Beckoning to summon forth Jarien and Sothos and slay them. Return to Bodley inside Blackrock Mountain with the recombined Lord Valthalak's Amulet and the Brazier of Beckoning.
The siblings, Jarien and Sothos, decided to join the Scarlet Crusade upon the disbanding of our mercenary company. They took the right piece of Lord Valthalak's amulet with them. Good riddance, I say, but we need that piece back.
From what we could gather, they were actually slain by Grand Crusader Dathrohan himself for failing the initiation rites inside his chamber in the Scarlet Bastion.
In any case, we... meaning you, are still going to need to retrieve that piece of the amulet.